I would agree with Manny Cav. There might also be an explanation to why they never change in appearance, but I must get into science mode.
Give me a second. Okay, physics, biology, chemistry, astronomy. I'm in science mode now. Prepare for scientific overload.
For the body to change in age, there must be hormonal activity. If there is a deficiency in hormones, then the body will not change as rapidly, which can make an individual seem younger than they really are. Therefore, my theory is that the gang has a damaged pituitary gland, specifically the ones that supply the body with hormones that cause the rapid growth that is known as adolescence.